In the course of investigations to find as gentle a method as possible for destroying deposits foreign to the body, such as, for example, kidney stones, gall stones, ureter stones or calcified tissue, there was no shortage of proposals and attempts to make use of light energy instead of other forms of energy, such as ultrasound and electrical energy, for this purpose. A possibility in this connection is to apply pulsed laser radiation directly to the surface of the concrement to be destroyed and consequently to destroy the latter thermally. A successful clinical trial of this method of destroying biliferous duct stones by means of flashlamp-pulsed light from a neodymium-YAG laser is reported in DMW 1986, 111, No. 31/32, page 1217. Here laser pulses having a duration of 2 ms were concentrated on the stones via a flexible 0.2 mm thick glass fiber which was supported in a special catheter, water and contrast agent being instilled at the same time. The quantity of energy required in this thermal method is high and it is hardly possible to protect the environment of the operation area against a thermal stress.
It has also already been proposed to utilize the laser light absorbed at the surface of the stone for an electroacoustic effect, which would result in a lower energy requirement. A precondition for this is to use laser light of a wavelength which has only a low capability of penetrating the material of the stone. This applies to laser light in the visible range, for instance at 350 to 550 nm, it being necessary to match the wavelength within this range to the material of the concrement.
An apparatus for carrying out this method is described in WO 86/06269 in which a laser of the so-called "dyelaser" type is used. In addition to the disadvantage of having to match the wavelength of the laser light to the chemical nature of the concrement in order to be able to carry out the method with success, this method also has the further disadvantage of using laser light in the visible range which makes strong light filters, which impede the observation of the area of operation tremendously, necessary to protect the eyes of the operating doctor.
In "Energiewandler zur Steinzerstorung in den ableitenden Harnwegen des Menschen" ("Energy transducers for stone destruction in the human urinary discharge tracts"), Aktuelle Nephrologie 1 (1978), pages 138-144, H. Schmidt-Kloiber has already proposed using for this purpose a method in which, in the immediate vicinity of the concrement, light energy is converted into mechanical energy in the form of a cavitation bubble associated with the occurrence of shockwaves, said mechanical energy being responsible for the destruction of the concrement. For this purpose, pulsed laser light is fed via a light guide into the operation region and concentrated in the vicinity of the surface, not, however, at the surface of the stone to be destroyed, so-called laser-induced breakdowns which result in shockwaves and cavitation which effect the destruction of the stone, resulting from the high electric field strength produced in this process in the liquid environment of the stone, which environment is the result of continuous rinsing with rinsing liquid. Since the dispersion of the stone is effected in this case by the mechanical energy of the shockwaves and cavitation and is not based on an absorption of the laser light by the stone, this method, which bears the name laser-induced shockwave lithotripsy, has the advantage that its success is independent of the chemical nature of the stone and it is not necessary to rely on laser radiation of a certain wavelength. Laser light in the infra-red region can therefore be used to carry out this method, and this entails great advantages. The basic physical and technical aspects of this method and its practical execution have been described by the inventors H. Schmidt-Kloiber and E. Reichel, and also by H. Schoffmann in Biomed. Technik 30 (1985), 173-181, and also by the inventor H. Schmidt-Kloiber in Aktuelle Nephrologie 1 (1978), pages 138-144, which is incorporated therein by reference. Equipment set ups for carrying out said method are described, for instance, in Austrian Patent Nos. 382,777 and 380,634.
As was disclosed by H. Schmidt-Kloiber and E. Reichel in Acustica, vol. 54 (1984), page 284, a laser-induced breakdown takes place in liquids only at high pulse energies for any laser emission. With decreasing laser pulse energy, the frequency of the breakdowns decreases until a threshold energy is reached below which virtually no breakdowns can any longer be achieved. These results are of importance since, on the one hand, the effectiveness of the method depends on a good utilization of the laser pulses delivered, but on the other hand, the intensity to be transmitted is limited by the need to use a light guide since the intensity cannot be chosen so high that laser-induced breakdowns already occur in the light guide.
As a result of the above cited paper by H. SchmidtKloiber and E. Reichel in Acustica, vol. 54, it is also already known that, in in-vitro tests which were carried out with water, an aqueous solution of 27 g of sorbitol and 5.4 g of mannitol per liter and a 0.9% saline solution, the threshold energy was lowest if saline solution was used and the breakdown frequency for this solution increased more quickly with increasing pulse energy than for the other two liquids tested. These results were obtained with an experimental arrangement in which the pulsed laser light was focussed via a convergent lens in a cell which contained the experimental liquid. For an in-vivo application of the method of stone destruction, however, it has to be borne in mind that the transport of light through an optical guide signifies an energy loss so that more energy has to be presented than is necessary for an adequate breakdown frequency in accordance with the in-vitro test described above. Here, however, limits are imposed since not only must the threshold intensity for a breakdown in the light guide material not be exceeded, but also the diameter of the light guide is limited to about 1 mm for reasons of usability in the body (Biomedizinische Technik 30 (1985), page 177).
Realistic tests using light guides which were carried out within the scope of experiments which result in the present invention showed that, if laser light situated in the infra-red region is used, the threshold energy of 0.9% saline solution is still too high to achieve a usable breakdown frequency. The object therefore existed of finding liquid media with which a high breakdown frequency is achieved in physiologically compatible concentrations using a light guide. Since the breakdown is associated with the development of a brightly luminous plasma (gas bubble), it can also be detected visually and the shockwave (effect) resulting therefrom can be qualitatively assessed.
Surprisingly, it was possible to find that the compounds of the ferrous metals, iron, cobalt and nickel, and of the alkaline-earth metals magnesium and calcium have a considerably lower threshold energy than for sodium chloride in aqueous solution in much lower concentrations than for the latter and that in these solutions, said compounds are capable, in a special manner, of electron generation which expresses itself in intense plasma formation which can be recognized from an intense plasma luminescence. Said plasma luminescence is not obtained at all with 0.9% saline solution under identical experimental conditions.